- From: Eric Prud'hommeaux <eric@w3.org>
- Date: Sat, 8 Oct 2011 18:02:16 -0400
- To: Andy Seaborne <andy.seaborne@epimorphics.com>
- Cc: public-rdf-wg@w3.org
* Andy Seaborne <andy.seaborne@epimorphics.com> [2011-10-08 17:21+0100] > > > On 07/10/11 16:04, Eric Prud'hommeaux wrote: > >* Sandro Hawke<sandro@w3.org> [2011-10-07 10:35-0400] > >>On Fri, 2011-10-07 at 13:48 +0100, Andy Seaborne wrote: > >>>>Okay, that's enough for now. Give me a +1 if you think this is headed > >>> > in a useful direction. > >>> > >>>I like something like this as a pattern of good practice (well, 2 > >>>patterns). I don't agree with forcing the 4th column to have a specific > >>>meaning given all the other deployed uses we have now collected. > >> > >>Yeah.... There is a middle ground where some datasets use Web > >>semantics and some don't. I see your point that we can't just force > >>people to change -- we can't say the thingsthey've been saying now means > >>something else. > >> > >>Maybe we can have a way to flag which datasets are using Web semantics, > >>and allow market pressures to work? Like, where we do a new mime type > >>for a multigraph syntax, we could add this. And maybe it's something > >>we can flag in SPARQL service description. > >> > >>>On one points: > >>> > >>>I don't see why > >>> > >>><http://example.org> {<s> <p> <o> . } > >>> > >>>should mean it is ONLY that triple rather than CONTAINS that triple. If > >>>the data publisher wants to say "and that's all" then they should say so > >>>as an additional fact. The converse of "it's closed by default" is > >>>harder to see how to allow it to be open sometimes. > >>> > >>>For a large graph, and you only need to talk about a small subset, the > >>>deployment issues. Consider dbpedia. > >>> > >>>(I also want to see the same change in TriG for concatenation of files) > >> > >>It seems to me that it's easy to go from complete to incomplete, just > >>using a subgraph predicate. Let's say we want to say G1 is the graph > >>with only<s> <p> <o> and G2 is a graph with that triple and maybe other > >>stuff. I'd say: > >> > >> G1 {<s> <p> <o>. } > >> { G1 r:subgraphOf G2. } > >> > >>But I don't see how to communicate G1 the way you're talking about. How > >>do you say "and that's all"? > > > >Imagining Trig used for both update and patch, I see it as specified > >by the protocol. CONSTRUCT ?g { ?s ?p ?o } would give me the results > >of a query substituted into a named graph pattern. A reply to a GET > >would give me a complete resource ("and that's all"). A diff propa- > >gation would could look like: > > -<G1> { _:s1<p> <o0> } > > +<G1> { _:s1<p> <o1> } > >which means there were already some<G1> triples and we've only > >changed one of them. The use you want to define is, I believe, > >characterized by GET<G1>, but I think the mapping of graph > >names to sets of triples is useful in other places with other > >presumptions of completeness. > > SPARQL Update allows various ways of treating a change: > > > > # if you want "replace", clear the destination first: > CLEAR <G1> ; > INSERT DATA { GRAPH <G1> { <s> <p> <o> } } > > or a change: > DELETE DATA { GRAPH <G1> { <s> <p> <o0> } } > INSERT DATA { GRAPH <G1> { <s> <p> <o1> } } Yeah, this could be thought of as a transfer of two datasets and I have a hunch we'll see more clever uses if we don't demand that a "dataset" is a total state but a bag to be exploited by protocols. I don't have more than a hunch there, but I guess there's some history to say that over-specifying leads to re-negotiation. > Andy > > > > > > >> -- Sandro > >> > >> > >>> Andy > >>> > >>>On 07/10/11 03:04, Sandro Hawke wrote: > >>>>Here's a proposal for what the fourth column should mean. It's kind of > >>>>obvious, and I think it's how many of us just assumed Named Graphs were > >>>>supposed to work. But I don't think it's been written down in a form > >>>>we can use, so here it is, in a first draft. > >>>> > >>>>I haven't really tried to motivate this, but one thing it does is allow > >>>>folks to refer to a graphs using just one URI. As [1] points out rather > >>>>painfully, as things stand now, you need multiple URIs just to identify > >>>>each g-box (and thus g-snap). (That is, you need to say which sparql > >>>>endpoint you're talking about, and then which graph within its > >>>>dataset.) > >>>> > >>>>My starting question was: what is the relationship between the IRI (the > >>>>"graph name") and its associated g-snap in an RDF Dataset. This > >>>>applies to the dataset backing any SPARQL end point, as well as the > >>>>dataset serialized in any multigraph syntax, like TriG or N-Quads. > >>>>Another way to look at it: what does it mean to assert a TriG > >>>>document? If you send me the TriG Document "<a> {<s> <p> <o> }", and > >>>>I trust you, what do I now know? > >>>> > >>>>Richard, I think, has been arguing for a minimalist position, > >>>>answering "nothing", or "it depends on out-of-band agreements". This > >>>>"Web Semantics" proposal is an alternative. > >>>> > >>>>=== Proposal > >>>> > >>>>The idea here is to make the relationship between the URI and the > >>>>graph be the standard Web naming relationship, similar to what we all > >>>>use for Web pages. When you dereference the URI, you get the graph. > >>>> > >>>>This has the feature of being, to some extent, observable. Just like > >>>>triples are claims about some domain of discourse, quads become claims > >>>>about idealized Web dereference behavior. > >>>> > >>>>Specifically: Consider a "graph naming" to be the association of a > >>>>graph name N with a graph G. For the graph naming to hold, every > >>>>successful dereference of N yielding an RDF graph must yield G. For a > >>>>dataset D to hold, its default graph must hold (as normal in RDF) and > >>>>every graph naming pair in D must hold. > >>>> > >>>>Example 1: This dataset > >>>> > >>>> <http://example.org> {<s> <p> <o>. } > >>>> > >>>>means that if anyone is able to dereference "http://example.org" > >>>>and obtain an RDF graph serialization, the serialized graph will > >>>>consist of the single triple,<s> <p> <o>. Failure to dereference > >>>>does not make the graph naming untrue, but a successful dereference > >>>>yielding a different graph does. > >>>> > >>>>Example 2: This dataset can never be true: > >>>> > >>>> <http://example.org> {<s> <p> 1. } > >>>> <HTTP://example.org> {<s> <p> 2. } > >>>> > >>>>... since one cannot get different results dereferencing URIs that > >>>>differ only in the case of the scheme component (as per RFC 3986). > >>>> > >>>>Example 3: This dataset: > >>>> > >>>> <tag:hawke.org,2010-10-06:eg1> {<s> <p> <o>. } > >>>> > >>>>cannot be tested using Web protocols, since the "tag" URI scheme is > >>>>(by design) not dereferenceable. Whether it is true or false cannot > >>>>be determined experimentally. > >>>> > >>>>==== Temporal Context > >>>> > >>>>How can we say: > >>>> > >>>> <http://example.org> {<s> <p> <o>. } > >>>> > >>>>if we suspect that "http://example.org" might serve some other content > >>>>tomorrow? > >>>> > >>>>The answer is that datasets often need temporal qualification just > >>>>like RDF graphs do. It's just like saying in RDF: > >>>> > >>>> <http://example.org/Alice> foaf:age 25. > >>>> > >>>>One solution for foaf:age triples is to include triples like: > >>>> <> dc:temporal "2011-10-06"^^xs:dateTime. > >>>> > >>>>and that can be done in datasets as well, using the default graph. > >>>>More work is needed on this, but I'm pretty sure this proposal can use > >>>>whatever solution people come up with for RDF and doesn't make matters > >>>>much worse than they are already. > >>>> > >>>>==== Practical Deployment Choices > >>>> > >>>>Any system which maintains a dataset (eg a sparql endpoint) or > >>>>generates multigraph documents like TriG has to do one (or more) of > >>>>the following: > >>>> > >>>>1. Use new non-dereferenceable graph names. These could be tag or > >>>> uuid URIs, or http URIs in your own name space which you choose to > >>>> leave 404. > >>>> > >>>>2. Use your own dereferenceable graph names, perhaps relative to the > >>>> endpoint or TriG document URI. If you do serve RDF content at > >>>> those URIs, it MUST be the same content (give or take stated time > >>>> lag). > >>>> > >>>>3. Use someone else's graph names. Here, the key thing is temporal > >>>> metadata. You have to decide what you want (copy once vs > >>>> synchronize with what accuracy) and (somehow) share that temporal > >>>> metadata. > >>>> > >>>> > >>>>... > >>>> > >>>>Okay, that's enough for now. Give me a +1 if you think this is headed > >>>>in a useful direction. > >>>> > >>>> -- Sandro > >>>> > >>>>[1] http://www.w3.org/2011/prov/wiki/Using_named_graphs_to_model_Accounts > >>>> > >>>> > >>> > >>> > >> > >> > >> > > > -- -ericP
Received on Saturday, 8 October 2011 22:02:58 UTC